You might visit Rick Ross's website to browse up on the subject. The situation sounds like a number of well known groups but it could also be a small group that is under the radar. It would be very useful if your boss could find out the name of the group. then a web search might reveal all sorts of useful info. I would also check with law enforcement to see if they have any info on the group, group's members/leaders.
while being over 18 does limit some of the potential actions your boss can take it doesn't completely preclude everything. your boss could involve law enforcement if there is reason to question how willingly she has gone with the group or whether she might be subject to duress, intimidation, force, threats, etc. finding out her location and some info about the group, might at least enable your boss to arrange for a welfare check by the local authorities on her (which he could probably accompany them if he arranged it).
As an alternative, it may be possible to file in court to get a court order of examination to determine whether she is mentally competent. There are lots of hoops to jump through to get the order but if successful, that might at least lead again to an interview and examination by a professional - giving again an opportunity to intervene.
Should he manage to meet with her, in no way, should your Boss try to kidnap her or keep her some place against her will, etc. the way the old deprogrammers used to do as that would lead to bigger problems for him including possibly being criminal.
best of luck. let us know how this situation turned out...